Stock-Based Compensation - Summary of Compensation Expense Recognized With Regard to Stock Options Granted to Non-employee Consultants Including Effect of Re-measurement of Fair Values of Options (Detail) - Non-employee Consultants [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	$ 133	$ (4)	$ 300	$ 179
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	93	3	121	146
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	$ 40	$ (7)	$ 179	$ 32